clockvector: bugfix - fixup vector initialization, size

clockvector: bugfix - fixup vector initialization, size

This fixes a few bugs in ClockVector:

- The parent-less vector is the initial-thread creation, so set its vector
  clock[0] = 1
- The clock vector's data should be initialized to 0
- A clock vector cannot determine its size simply by the size of its parent (+1
  if 'act' is THREAD_CREATE); other threads could have been created, giving an
  inconsistent result here. Instead of trying to be smart about calculating
  num_threads from the parent, then, I just ask for it globally.